your dismounted units will fly instead on sitting on horse, actually...Alex has more slots available, not sure how much, (may be unlimited?) but much, so go on. This should help http://www.twcenter.net/wiki/Adding_Units_-_RTW all necessary links are there. Also don't forget to add new unit cards and entry into EDB to recruit them in buildings. And you shouldn't forget to backup all the stuff.
PS
and don't forget to make new sprites, you don't want to see your dismounted units sitting on horses when the camera is far, isn't it?
BTW, Alex model limit? just count every entry on LOTR-TW DMB, they've used almost all Alex limit I think
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